Capacitor Doubt

Discussion in 'General Electronics Chat' started by imraneesa, Feb 20, 2015.

  1. imraneesa

    Thread Starter Member

    Dec 18, 2014
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    • Input voltage: 2.6V to 10V Max (Limited by input capacitor)
    • Output voltage: 1.25V to 10V (Limited by output capacitor)

    I saw this in some webpage. that is regarding the laser diode driver.

    my question is how can we limit the output using capacitor and what is the formula for that.
    if my input is 7.4v and I want the output to be 3.5v how can I limit it using capacitor on the output.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Capacitors do not limit the voltage.
    Show us the schematic you are talking about.

    Bertus
     
  3. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    he did not give any schematic. he is just selling linear drivers using LM1085. in that he told the output is 1.25v to 10v (limited by output capacitor).
    so capacitor will not limit the voltage right?
     
  4. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    LM1084 based linear driver: $8 shipped CONUS! (INTL available at extra cost)
    • Low drop out (~0.8V)
    • Input voltage: 2.6V to 10V (Limited by input capacitor)
    • Output voltage: 1.2V to 10V (Limited by output capacitor)
    • Output current: 50mA to 2000mA (5A max)
    • 2 x lithium ion batteries recommended
    • Outputs over 1A heat sink is a must! Both IC and resistors!
    • Dimensions (11mm x 17mm)
    • Continuous ground
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    The LM1084 is a voltage regulator.
    I assume the laser diode would need constant current, wich the LM1084 will not deliver.
    Do you have more details?

    Bertus
     
  6. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    This is the schematic they are using.
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In that schematic the LM is connected as constant current source.
    I have a schematic from the LM317 datasheet:

    [​IMG]

    Bertus
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the limit is the maximum voltage rating of the capacitor. the voltqge rating of the cap has nothing to do with the regulator.
     
    planeguy67 likes this.
  9. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    ok so capacitor has nothing to do with the limiting. I know about that. but just was in doubt when I saw those statements. so I thought to clarify it here. thank you so much friends. I am looking for 1.8A driver with less heat sink. if you have any good ic's for this purpose please let me know.
     
  10. bertus

    Administrator

    Apr 5, 2008
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  11. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
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    OK sir. I will look into that. And will come back if any doubts. Thank you.
     
  12. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    The limits referred to may have been a result of the voltage rating of the input and output capacitors. Just a guess.
     
  13. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    lm2679 minimum input voltage is 8v. so I cannot use that. I am trying to use LM3409. I will attach the schematic very soon for your review and correction.
     
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